This original Leaflet describes the history of the fungus, disease symptoms and control measures of Keithia disease of thuja plicata in Western red ceder in Britain. Under conditions in Britain the fungus appears to overwinter mainly in the form of ascospores attached to the needle surface. The initial symptoms of infection are the browning of individual needles scattered over the shoots. The general pattern of disease development is a high level of infection in late April and early May with a fluctuating level of attack through the summer and autumn, depending on climate conditions. As the disease is not seedborne, the introduction of infection is confined to airborne spores. The method of control, currently practiced by the Forestry Commission, is a natural development linked to the production of plants from a series of isolated nurseries. Effective chemical control has been achieved recently with cycloheximide fungicides, although an effective and safe treatment can now be prescribed, proprietary formulations are not yet generally available in Great Britain. By a programme of cross-breeding it is hoped to produce one or more cultivars of red cedar with good silvicultural characteristics and a high degree of resistance.